At present, known dry developing processes in various electrostatic copying systems include a two-component developing system using a toner and a carrier such as an iron powder, and a one-component magnetic developing system using a toner containing a magnetic substance in the inside thereof without using the carrier, which are used in copying machines, printers, etc.
The developing system using the one-component magnetic toner does not require an automatic concentration controller necessary to a developing machine of the two-component developing system, so that maintenance such as carrier exchange is not required. This system has therefore come into use not only in low-speed small-sized copying machines and printers, but also in medium-speed or higher-speed copying machines, printers and plotters, so that a further improvement in performance has been expected.
The recent progress in digitization not only in the field of printers but also in the field of copying machines makes it possible to form finer latent images, and delicate gradation in small Chinese characters or dots has come to be able to be expressed. It has been therefore desired to faithfully develop highly fine latent images as it is, and studies have been conducted thereon. As a part thereof, the particle size of toners is minimized. Previously, toners having a volume average particle size within the range of from 9 to 11 .mu.m have been used. Recently, however, toners having a volume average particle size of from 4 to 9 .mu.m are becoming the main current.
Although the minimization of the particle size of toners provides the excellent effect, it generally has the drawback of being poor in transfer performance. That is, the minimization of the particle size of toners to 4 .mu.m to 9 .mu.m gives very high image quality, but deteriorates transfer and cleaning abilities. When fine particles having high hardness such as metal and non-metal oxides are added to the surfaces of the toner particles as employed in conventional methods for improving cleaning ability, photoreceptors are unevenly shaved not only to raise problems with regard to maintaining property and resource saving, but also to be affected by changes in potential due to shaving, thereby deteriorating high image quality such as excellent gradation. Further, wax-containing toners reduce the wear of photoreceptors and improves fixing property by suppressing smudges, in which toners are transferred to white paper by fracture after fixing, hot offset, and the like, but deteriorates filming. This makes more difficult to carry out a cleaning process of toners remaining on the surfaces of the photoreceptors after transfer.
Further, when a charging roller or a transfer roller to which a high voltage has been applied is used in a charging stage or a transfer stage, a residue on the surface of a photoreceptor is brought into contact with the charging roller or the transfer roller under pressure. As a result, a toner, toner components, a fluidity imparting agent added to the toner, etc. remaining on the photoreceptor are transferred to the charging roller or the transfer roller to contaminate the surface thereof. Consequently, essential functions such as the charging function and the transfer function are prevented, thus resulting in a decrease in image density in long-term use and in generation of image unevenness.
Furthermore, when the one-component magnetic toner is used in a developing stage, it is considered to be more easily cleaned than a nonmagnetic toner because of the action of a relatively free magnetic substance. However, for example, when a cleaning blade such as a rubber blade frequently employed is used, there cause problems, for example, the toner components remaining on the surface of the photoreceptor are scraped, thereby damaging the surface of the photoreceptor and causing adhesion of the toner components. In particular, when wax is added to improve smudges, in which toners are transferred to white paper by fracture of parts of fixed images scrubbed and the hot offset phenomenon, adhesion of the toner easily takes place. Further, wear of the cleaning blade itself also occurs to bring about a reduction in cleaning performance, which causes black streaks, image distortion, tailing, etc. in images. They are unfavorable for providing high image quality.
In order to improve the above problems, attempts have previously been made to add fine inorganic particles such as metal/non-metal oxide particles to the surfaces of the toner particles. For example, JP-A-53-81127 (the term "JP-A" as used herein means an "unexamined published Japanese patent application) discloses the technique of adding an abrasive such as CeO.sub.2 to a toner and scraping an adhered material on a photoreceptor. JP-A-61-236560 also discloses the technique of adding CeO.sub.2 as an essential component and further adding a rare earth element compound for the same purpose. In these cases, however, when the photoreceptor has an organic photoconductive layer, the abrasive having a higher hardness than the surface of the photoreceptor gets into between the blade and the surface of the photoreceptor to thereby shave the surface layer. Problems are therefore encountered with regard to maintaining property and life, although adhesion of the toner components is inhibited. Further, the surface layer is unevenly shaved by variously sized CeO.sub.2, so that deterioration of image quality by changes in potential takes place. As the technique for suppressing these troubles, JP-A-61-231564 discloses addition of abrasives having low hardness such as carbonates and sulfates of alkaline earth metals, rare earth elements and transition metals, tri-iron tetroxide and clay minerals having a layer structure. However, there is a fear of poor cleaning due to low hardness and a reduction in gradation reproduction due to electrostatic charge leak caused by water inclusion property of clay minerals themselves. Further, although a photoreceptor mainly formed of amorphous silicon can prevent image flowing, the charging level under the circumstances of low temperature and humidity is high, and the charge-up phenomenon is largely observed. Problems arise with regard to image density maintaining property and ghosts in the one-component developing system. On the other hand, JP-A-1-204068 discloses addition of cerium fluoride giving attention to its low surface energy. However, fluorine content of cerium fluoride is too high, so that it has the problem of poor cleaning. Further, since the negative charging property becomes too high, a problem is encountered with regard to image density maintaining property.
As described above, the conventional techniques have various problems, which have not been solved yet at present.